As is known to the persons skilled in the art, the aforementioned type of chain presents considerable production problems due to its particular linkage pattern, which requires engagement of more than one link with others with small reciprocal play. One problem is to ensure an adequate connection between links after closure, to avoid failure of the chain even under light stress. Another problem is to place the extremities of each link in the most convenient position for the subsequent soldering phase. To obtain these results the extremities of the links are shaped so as to reduce their dimensions, to permit easier mutual penetration, and to provide a greater support surface for the soldering material and thus greater strength in the finished chain. To these purposes two types of machines for automatic production of this type of chain have been proposed, as alternatives to the traditional semi-manual methods.
A first type of these machines consists of two distinct, separate units, one for producing the link and the other for assembling the chain. A coil of filiform material is formed (generally a closed coil; that is, one in which the single turns are in contact with each other) and the single links are separated from the coil by milling with a milling-tool operating under the coil and tangentially to it to produce symmetrical, substantially nail-shaped extremities of each link. With the aid of vibration feeders or other suitable systems, the links thus formed are fed to a conventional linking machine equipped with pincers for engaging the links one to the others and for closing them.
A second type of machine for the manufacture of rope chain type chains operates by shearing off straight pieces of filiform material of suitable length with a punch so shaped as to obtain at the same time the desired contouring of the extremities of the piece cut. The piece is then taken up by internally-grooved pliers and forced against a transversal pin to obtain an open ring thus forming an open link with opposed, contoured extremities, housed in the groove of the pliers. The link is then joined in traditional manner to the already connected links, and closed over them to form the chain.
The chains produced with the machines described above, despite the shape of the extremities of the links, do not possess satisfactory strength and therefore they tend to come apart before soldering; this is due also to some springback in the material itself. To reduce this risk, the chains are frequently subjected to a final hammering which inevitably leaves tool marks, sometimes quite visible, on the surface of the links.
In practice, due to the structure of the chain, it is not possible to obtain, by means of pliers, link closure for a secure connection, even when the extremities are very sharply shaped, as obtained with the first type of the above described machine. The pliers, due to their very dimensions, cannot grip the link more than a certain amount without interfering with the other, neighboring links or with the pincers that support the already-formed length of chain. It is clear that, the smaller the links to be closed, the greater is the inconvenience.
On the other hand, it is evident that is not possible to reduce the lateral ribs of the pliers, in order to render them less bulky, below a certain limit without jeopardizing their strength.
In the case of links produced with the second type of machine, a less sharp shape of their ends is obtained and, moreover, the ends of the links are symmetrically opposed (the ends of the links obtainable by means of the first type of machine, being taken from a coil, are staggered one with respect to the other). This makes it impossible to completely close the link even after hammering and, moreover, provides only a very small useful surface for soldering: thus, even after this operation, the strength of the chain is relatively low.
The first type of machine as hereinbefore described permits obtaining a shape of the extremities of the links better suited for closure than those obtainable by shearing but this machine is, on the other hand, much more complicated mechanically, because it is composed of two separate operative units connected by intermediate link transport means. Finally, the problem of recovery of the material removed as dust by the milling tool, cannot be ignored. Dust recovery is essential in goldsmithery for economic reasons and anyway for health reasons in the working environment, where non-precious material are handled. Suitable aspiration and transport devices for collection and recovery of the aforementioned dusts must therefore be installed.